The present invention is concerned with the utilization of non-chloride additives, namely sodium thiocyanate (NASCN) and/or calcium nitrate (Ca(NO3)2), for shortening the setting times of concrete ready-mixes. Further, the invention is also concerned with the utilization of three industrial by-products; namely, Class-F fly ash, Class-C fly ash, and blast furnace slag in general purpose concrete-making compositions.
When finely divided or pulverized coal is combusted at high temperatures, for example, in boilers for the steam generation of electricity, the ash, consisting of the incombustible residue plus a small amount of residual combustible matter, is made up of two fractions, a bottom ash recovered from the furnace or boiler in the form of a slag-like material and a fly ash which remains suspended in the flue gases from the combustion until separated therefrom by known separation techniques, such as electrostatic precipitation. This fly ash is an extremely finely divided material generally in the form of spherical bead-like particles, with at least 70% by weight passing a 200 mesh sieve, and has a generally glassy state resulting from fusion or sintering during combustion. As recognized in the American Society of Testing Materials (ASTM) specification designations C618-00 entitled “Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Portland Cement Concrete” and D5370-96 entitled “Standard Specification for Pozzolanic Blended Materials in Construction Application,” fly ash is subdivided into two distinct classifications; namely, Class-F and Class-C. The definitions of these two classes given in the aforementioned ASTM specifications are as follows:
“Class-F—Fly ash normally produced from burning anthracite or bituminous coal that meets the applicable requirements for this class as given herein. This class fly ash has pozzolanic properties.
Class-C—Fly ash normally produced from lignite or subbituminous coal that meets the applicable requirements for this class as given herein. This class of fly ash, in addition to having pozzolanic properties, also has some cementitious properties. Some Class-C fly ashes may contain lime contents higher than 10%.”
The latter reference to “pozzolanic properties” refers to the capability of certain mixtures that are not in themselves cementitious, but are capable of undergoing a cementitious reaction when mixed with calcium hydroxide in the presence of water. Class-C fly ash possesses direct cementitious properties as well as pozzolanic properties. ASTM C618-00 is also applicable to natural pozzolanic materials that are separately classified as Class N but are not pertinent here.
As the above quotation from the ASTM specification indicates, the type of coal combusted generally determines which class fly ash results, and the type of coal in turn is often dependent upon its geographic origin. Thus, Class-C fly ash frequently results from the combustion of coals mined in the Midwest United States; whereas Class-F fly ash often comes from combustion of coals mined in the Appalachian region of the United States. The ASTM specification imposes certain chemical and physical requirements upon the respective fly ash classifications which are set forth in U.S. Pat. No. 5,520,730, the disclosure of which is incorporated herein by reference.
Blast furnace slag is a by-product of the production of iron in a blast furnace; silicon, calcium, aluminum, magnesium and oxygen are the major elemental components of slag. Blast furnace slags include air-cooled slag resulting from solidification of molten blast furnace slag under atmospheric conditions; granulated blast furnace slag, a glassy granular material formed when molten blast furnace slag is rapidly chilled as by immersion in water; and pelletized blast furnace slag produced by passing molten slag over a vibrating feed plate where it is expanded and cooled by water sprays, whence it passes onto a rotating drum from which it is dispatched into the air where it rapidly solidifies to spherical pellets. In general the glass content of the slag determines the cementitious character. Rapidly cooled slags have a higher glass content and are cementitious; slowly cooled slags are non-glassy and crystalline and, thus do not have significant cementitious properties.
The quantities of these by-product materials that are produced annually are enormous and are likely only to increase in the future. As petroleum oil as the fuel for the generation of electricity is reduced because of conservation efforts and unfavorable economics, and as political considerations increasingly preclude the construction of new nuclear power electrical generating facilities, or even the operation or continued operation of already completed units of this type, greater reliance will necessarily fall on coal as the fuel for generating electricity. As of 1979, the volume of Class-F fly ash that was available then was estimated to be about five times what could be readily utilized. The estimated annual total production of coal ash in the U.S. is about 66.8 million tons, while the annual total coal ash sales in the U.S. is only about 14.5 million tons. Further, in Canada, the recovery of copper, nickel, lead and zinc from their ores produces over twelve million tons of slag per year, which is usually accumulated near the smelters with no significant use. Obviously, there is an urgent and growing need to find effective ways of employing these unavoidable industrial by-products since they will otherwise collect at a staggering rate and create crucial concerns regarding their adverse environmental effects.
Various proposals have already been made for utilizing both types of fly ash. According to Lea (1971), The Chemistry of Cement and Concrete, Chemical Publishing Company, Inc., page 421 et seq., fly ash, i.e., Class-F type, from boilers was first reported to be potentially useful as a partial replacement for Portland cement in concrete construction about 50 years ago, and its utilization for that purpose has since become increasingly widespread. It is generally accepted that the proportion of Portland cement replaced by the usual fly ash should not exceed about 20% to avoid significant reduction in the compressive strength of the resultant concrete, although some more cautious jurisdictions may impose lower limits, e.g., the 15% maximum authorized by the Virginia Department of Highways and Transportation (VDHT). As described in Lea on page 437, the substitution of fly ash tends to retard the early rate of hardening of the concrete so that the concrete exhibits up to a 30% lower strength after seven days testing and up to a 25% lower strength after 28 days of testing, but in time the strength levels normalize at replacement levels up to 20%. Increasing the substitution quantity up to 30% gives more drastic reduction in the early compression values as well as an ultimate strength reduction of at least about 15% after one year.
The limited substitution of fly ash for Portland cement in concrete formulations has other effects beyond compressive strength changes, both positive and negative. The fly ash tends to increase the workability of the cement mix and is recognized as desirably reducing the reactivity of the Portland cement with so-called reactive aggregates. On the other hand, fly ash contains a minor content of uncombusted carbon that acts to absorb air entrained in the concrete. Because entrained air desirably increases the resistance of the hardened concrete to freezing, such reduction in entrained air is undesirable, but can be compensated for by the inclusion as an additive of so-called air-entraining agents.
Dodson, et al. in U.S. Pat. No. 4,210,457, while recognizing the accepted limit of 20% replacement with fly ash of the Portland cement in concrete mixes, proposes the substitution of larger amounts, preferably 50% or more, of the Portland cement with particular selected fly ashes having a combined content of silica, alumina and ferric oxide content, less than 80% by weight, and a calcium oxide content exceeding 10%, based on five samples of such ashes, varying from about 58-72% combined with a calcium oxide range of about 18-30%. Six other fly ash samples that are not suitable at the high replacement levels of 50% or more were shown to vary in the combined oxide content from about 87-92% and in calcium oxide content from about 4% to about 8%. Evaluating these values against the ASTM C618-00, one observes that the acceptable fly ashes come under the Class-C specifications, while the unacceptable ashes fell within the Class-F specification. Thus, Dodson, et al. in effect establish that Class-C fly ashes are suitable for substantially higher levels of replacement for Portland cement in concrete mixes than are Class-F fly ashes, and this capacity is now generally recognized, with Class-C fly ashes being generally permitted up to about a 50% replacement level while maintaining the desirable physical properties of the concrete, especially compressive strength.
In U.S. Pat. No. 4,240,952, Hulbert, et al. while also acknowledging the generally recognized permissible limit of Class-F fly ash replacement for Portland cement of 20%, propose replacement of at least 50% and up to 80%, provided the mix contains as special additives about 2% of gypsum and about 3% of calcium chloride by weight of the fly ash. The fly ash described for this purpose, however, was a Class-C fly ash analyzing about 28% calcium oxide and combined silica, alumina and ferric oxide content of about 63%. With up to 80% of this fly ash and the specified additives, compressive strengths comparable to straight Portland cement were said to be generally achievable. In one example using 140 pounds Portland cement and 560 pounds of fly ash (20:80 ratio) with conventional amounts of coarse and fine aggregate, and water and including the requisite additives, compressive strengths tested at 3180 psi for 7 days, 4200 psi for 14 days and about 5000 psi at 28 days.
In U.S. Pat. Nos. 4,018,617 and 4,101,332, Nicholson proposed the use of mixtures of fly ash (apparently Class-F in type), cement kiln dust and aggregate for creating a stabilized base supporting surface replacing conventional gravel or asphalt aggregate stabilized bases in road construction wherein the useful ranges were fly ash 6-24%, CKD (cement kiln dust) 4-16% and aggregate 60-90%, with 8% CKD, 12% fly ash and 80% aggregate preferred. Compressive strength values for such measures as revealed in the examples varied rather erratically and generally exhibited only small increases in compressive strength over the 7 to 28 day test period. Among the better results were for the preferred mixture wherein the values increased from about 1100 psi at 7 days to 1400 psi at 28 days. The addition of a small amount of calcium chloride improved those values by about 200 psi. On the other hand, the addition of 3% of lime stack dust recovered from a lime kiln significantly reduced the results to about 700 psi at 7 days to 900-1300 psi at 28 days. Elimination of the aggregate reduced the strength to a fraction of the values otherwise obtained, a mixture of 12% CKD and 88% fly ash alone showing strength values of only about 190-260 psi over the 28-day test period. Similarly, the choice of a finely divided aggregate such as fill sand resulted in about the same fractional level of strength values in the range of about 140-230 psi. A combination of finely divided and coarse aggregate in approximately equal amounts reduced the compressive strength values by about 50% with virtually no change over the test period, giving values ranging from 650-750 psi, except where 1% of Type 1 Portland cement was included which restored the strength values to about their original level, except at the initial 7 days period where the strength values were about 800-900 psi, increasing at 28 days to about 1200-1600 psi. Curiously, the best strength results were attained when 11.6% fly ash was combined with 3.4% lime with the balance crushed aggregate, the CKD being omitted entirely, for which the strength values while starting at a lower level of about 850-950 at 7 days increased to about 1700 psi at 28 days.
The combination of fly ash and lime stack dust incidentally mentioned in the later patent was explored further by Nicholson in U.S. Pat. No. 4,038,095 which concerns mixtures of about 10-14% fly ash, about 5-15% lime stack dust with the balance aggregate in the range of 71-85%. Somewhat inexplicably, the compressive results reported here for such mixtures do not reach the high level specified in the first two aforementioned Nicholson patents, the strength values specified being only about 1000 psi with the more general levels well below that depending on particular proportions.
In U.S. Pat. No. 4,268,316, Wills, Jr. discloses the use of mixtures of kiln dust and fly ash as a replacement for ground limestone and gypsum for forming a mortar or masonry cement, using proportions of about 25-55% Portland cement, about 25-65% CKD and 10-25% fly ash. When these mortar formulations were mixed with damp sand in the proportions of about one part cement mixture to 2.5-3 parts sand, compression strengths comparable to those of standard masonry cement composed of 55% cement clinkers, 40% limestone and 5% gypsum were shown for mixtures containing 50% cement, 24-40% CKD and 15-25% fly ash. Inexplicably, in one example, when the cement content was increased to 55% with 35% CKD and 10-% fly ash, the compressive strengths dropped by about 30-40% at both the 7 day and 28 day ages to levels inferior to the standard material. As the cement content was decreased, with corresponding increases in the CKD, the compressive strength values dropped drastically. On the other hand, in another similar example, mixtures containing 55% cement, 35% CKD and 10% fly ash proved superior in compressive strength, particularly at the 28 day age, to mixtures containing 50% cement, 35% fly ash and 15% CKD, as well as other standard masonry cements containing 50% cement, 47% limestone and 3% gypsum. Indeed, strength values dropped about 40% for the mixtures having a 5% reduction in cement and a corresponding 5% increase in the fly ash to values definitely inferior to the standard cements. Similar variations were shown under laboratory test conditions for comparable 50/35/15 mixtures dependent on the source of the fly ash while under actual construction conditions for the same mixtures, compressive strength values were reduced by about 50% for both the conventional masonry cement containing 55% Portland cement and comparable mixtures within the patented concept. The fly ash here was preferably Class-F with Class-C materials being less desirable.
In U.S. Pat. No. 4,407,677, Wills, Jr. went on to teach that in the manufacture of concrete products such as blocks or bricks, the fly ash usually employed in combination with Portland cement therein could be replaced in its entirety by CKD with modest improvement in early compressive strength values for such products. Thus, at one-day and two-day tests compressive strength values of about 500-800 psi were shown, but were said to increase to about 1200 psi after 28 days. The mixes disclosed in Wills, Jr. '677 contained 0.4-0.9 parts cement, about 0.1-0.6 parts CKD and 10-12 parts aggregate combining both fine and coarse materials, such as expanded shale and natural sand in a weight ratio of 80/20. Masonry cements generally develop at least about 95% of their strength properties at 28 days age so that additional aging of the patented products would not be expected to result in any significant increase in their compressive strength values.
U.S. Pat. Nos. 5,520,730 and 5,266,111 of the present inventor, disclose a general-purpose concrete composition comprising Portland cement, Class-F fly ash, and CKD. The patents also disclose a “synthetic Class-C fly ash blend” comprising Class-F fly ash and CKD. These patents disclose the advantages of early strength of concrete; however, utilization of sodium thiocyanate and/or calcium nitrate as a cement additives are not disclosed.
U.S. Pat. No. 6,008,275 to Moreau et al. discloses a cement composition comprising greater than about 10% by weight of a pozzolanic cement replacement based on the weight of said hydraulic cement and cement replacement, and a polycarboxylate polymer. The composition can also contain an accelerator such as calcium nitrate and sodium thiocyanate. The pozzolanic replacement material can be fly ash, class C and/or class F; blast furnace slag; calcine clay; and natural pozzolan materials. The amounts of calcium nitrate and sodium thiocyanate used by Moreau et al. are impractical and do not meet ASTM specifications for pre-blend cement. Further, the compositions of Moreau et al. are not pre-blend cements and are wet concrete mixtures.
U.S. Pat. No. 4,606,770 to Gerber discloses a cement mix including hydraulic cement, aggregate, sufficient water to affect hydraulic setting of the cement, and an additive comprising an N-methylol amide, specifically, tri- or tetramethylolglycoluril alone or in combination with other accelerators, such as calcium nitrate and sodium thiocyanate. The additive is present in an amount sufficient to decrease the time necessary for the hardening of the concrete mix. The compositions of Gerber are not pre-blend cements and are wet concrete mixtures.
U.S. Pat. No. 5,531,825 to Gartner et al. discloses a cement set-accelerating admixture and compositions containing such admixtures. The admixture compositions comprise one or more nitroalcohols present in an amount effective to increase the set acceleration of a hydraulic cement composition. The invention further comprises one or more conventional accelerators, including calcium nitrate and sodium thiocyanate. Gartner et al. disclose only pastes, mortars and concrete compositions, but fail to mention a pre-blend composition.
None of the above patents disclose pre-blend cement compositions for fast settling times, meeting ASTM Designations C 595 specifications for pre-blend cements; therefore, there remains a need for pre-blend cement mixes with fast setting times, because the addition of fly ash or slag to concrete often results in slow setting. There are many advantages for fast setting times, such as allowing the concrete structure to be put into service sooner, thereby reducing labor cost, and allowing pre-cast concrete and concrete masonry manufacturers to achieve rapid form and mold turnover.